Hot dip zinc-aluminum coated steel sheets have been widely used in the fields of automobiles, consumer electronics, building materials and the like. A representative category of the coated steel sheets includes the following three types in order of aluminum (Al) content in coating bath.
(1) Galvannealed steel sheets (composition of coating bath: for example, 0.125 to 0.14 mass % Al—Zn)
(2) Galvanized steel sheets (composition of coating bath: for example, 0.15 to 0.25 mass % Al—Zn)
(3) Zinc-aluminum alloy coated steel sheets (composition of coating bath: for example, 2 to 25 mass % Al—Zn)
As described above, the hot dip zinc-aluminum coated steel sheets are steel sheets which are coated by using the coating bath including molten metal such as molten zinc and molten aluminum. In the coating bath, zinc (Zn) is the main ingredient, aluminum (Al) is added in order to improve coating adhesion and corrosion resistance, and substances such as magnesium (Mg), silicon (Si) and the like may be added in order to improve the corrosion resistance.
Hereinafter, the galvannealed steel sheet is referred to as “GA” and the coating bath for manufacturing the galvannealed steel sheet is referred to as “galvannealed bath (GA bath)”. The galvanized steel sheet is referred to as “GI” and the coating bath for manufacturing the galvanized steel sheet is referred to as “galvanized bath (GI bath)”.
When the above-mentioned hot dip zinc-aluminum coated steel sheets are manufactured, a large amount of inclusions called dross forms in the coating bath. The dross is made of intermetallic compounds of Iron (Fe) dissolved in the coating bath from the steel sheet and Al or Zn included in the coating bath (molten metal). Specific compositions of the intermetallic compounds are, for example, Fe2Al5 which represents top-dross and FeZn7 which represents bottom-dross. The top-dross may form in all of the coating bath (for example, GA bath, GI bath) for manufacturing the hot dip zinc-aluminum coated steel sheets. On the other hand, the bottom-dross only forms in the galvannealed bath (GA bath).
Since the specific gravity of the top-dross is smaller than that of the molten metal which is the coating bath, the top-dross flows in the coating bath, and finally rises to top surface of the coating bath. When a large amount of the top-dross flows in the coating bath, the top-dross accumulates on the surface of the roll in the coating bath, which may cause surface defects on the steel sheets. Also the flowing top-dross accumulates in grooves of the roll in the coating bath, which may cause roll-slipping and roll-idling because of the decrease in the apparent friction coefficient between the roll and the steel sheet. In addition, when a relatively large size of the top-dross adheres to the steel sheet, the quality of appearance of a product deteriorates and the product becomes off-grade in some cases.
On the other hand, since the specific gravity of the bottom-dross is greater than that of the molten metal which is the coating bath, the bottom-dross flows in the coating bath, and finally deposits on the bottom of the coating tub. When a large amount of the bottom-dross flows in the coating bath, in the same way as the top-dross, the bottom-dross causes problems such as defects in the roll in the coating bath, roll-slipping, roll-idling, remarkable deterioration of the quality of the appearance which results from its adhesion to the steel sheet, and the like. Moreover, the bottom-dross does not rise to the top surface and is not rendered harmless like the top-dross. The bottom-dross flows in the coating bath for a long time, and the bottom-dross, which deposits on the bottom of the coating tub once, reflows in the coating bath again by transition of the coating bath flow. Therefore, it can be said that the bottom-dross is more harmful than the top-dross.
In particular, when the sheet threading speed of the steel sheet dipped into the coating bath is accelerated in order to improve productivity of the coated steel sheets, the bottom-dross which deposits on the bottom of the coating tub rises in the coating bath due to the coating bath flow which is derived from high-speed threading of the steel sheet. The above-mentioned dross adheres to the steel sheet and causes the dross defects on the steel sheets, which results in a factor of degradation of the coated steel sheet. Therefore, hitherto, the sheet threading speed of the steel sheet was suppressed and the productivity had to be sacrificed in order to ensure the quality of the coated steel sheets.
To solve the above-mentioned problems caused by the top-dross and the bottom-dross, many suggestions have been made in the past. As shown below, the suggestions are commonly methods of sedimentation separation and flotation separation of the dross by using the difference in specific gravity between the coating bath and the dross.
For example, in Patent Document 1, dross removal equipment is suggested, in which molten zinc including the dross is transferred from a coating tub to a storage tub and the dross is separated by sedimentation and flotation by using the difference in specific gravity between the dross and the coating bath. In the equipment, the capacity of the storage tub is 10 m3 or more, the transfer volume of the molten zinc is 2 m3/hour or more, and a baffle plate is installed in the storage tub to divert the coating bath flow. However, in Patent Document 1, the dross removal effect is overestimated because of utilization of an equation which is applicable to the particle sedimentation in case of a relatively slow coating bath flow. In addition, although the harmful size of dross is defined as 100 μm or more in Patent Document 1, the dross defects which are recently regarded as the problem include defects which are derived from dross with a size of approximately 50 μm. In fact, a countermeasure with a greater effect than that of Patent Document 1 is necessary. On the contrary, in a method described in Patent Document 1, in order to remove the dross with the size of approximately 50 μm, the capacity of the storage tub needs to be 42 m3 or more, which is not practical because the equipment must be larger. Moreover, in order to minimize the equipment, since sedimentation velocity of the bottom-dross is slow, the countermeasure other than Patent Document 1 is necessary.
In Patent Document 2, a coating equipment is suggested, in which enclosing parts are installed in a coating tub and the rise of the bottom-dross is suppressed by sedimenting and depositing the bottom-dross underneath the enclosing parts. However, in a method described in Patent Document 2, the bath flow at an upper area in the coating bath increases with an increase in coating rate, so that the bath flow at a lower area in the coating bath also increases gradually. Thus, since the dross with small size does not sediment and flows back to the upper area with the coating bath flow, the dross removal efficiency is low. Moreover, in case of the coating tub with practical capacity (for example, 200 ton), the dross with small size flows back between the upper area and the lower area of the coating bath, grows with time passage, and finally sediments in the lower area. However, at the time, a large amount of the bottom-dross which grows up to size which is enable to sediment flows in the upper area and the lower area of the coating bath, so that the effect as the countermeasure against the dross defects is low. Moreover, although it is necessary to remove eventually the bottom-dross which deposited at the lower area, dross cleanup operation is substantially impossible if the enclosing parts exist. Since considerable time and effort are needed for dismantlement of the enclosing parts, it can be said that technology described in Patent Document 2 is not practical.
In the equipment suggested in Patent Document 3, a coating container is divided into a coating tub and a dross removal tub, and the molten metal in the coating tub is transferred to the dross removal tub by using a pump. Moreover, the dross is separated by the sedimentation in the dross removal tub and the purified bath flows back in the coating tub through opening portion provided for the coating tub. However, since a method described in Patent Document 3 is the method in which the dross is separated by simply using the difference in specific gravity between the dross and the bath, separation efficiency of the dross with small size is low and the dross flows back to the coating tub with the coating bath flow. Moreover, in case of the dross removal tub with practical capacity (for example, 200 ton), the dross with small size which is formed in the coating tub circulates between the coating tub and the dross removal tub with the coating bath flow, grows with time passage, and finally sediments at the dross removal tub. However, at the time, a large amount of the bottom-dross which grows up to size which is enable to sediment flows in the coating tub and the dross removal tub, so that it can be said that the effect of technology described in Patent Document 3 is low as the countermeasure against the dross defects.
In addition, in a coating equipment suggested in Patent Document 4, the coating bath in a coating pot is transferred to a crystallization pipe, and is cooled and heated repeatedly several times in the crystallization pipe. Thereby, the dross is grown and removed, and the purified bath is reheated in a reheating tub and returned to the coating pot. Moreover, in a coating method suggested in Patent Document 5, a sub pot is additionally installed in a coating pot. The molten metal which includes the bottom-dross is transferred from the coating pot to the sub pot, the bath in the sub pot is held at higher temperature than that of the coating pot, and Al concentration is increased 0.14 mass % or more. Thereby, the bottom-dross in the coating bath is transformed into the top-dross, and the top-dross is removed by the flotation separation.